1. Field of the Invention
The present invention relates to a dielectric ceramic composition suitable for capacitors used in medium and high voltage applications, for example, such as a snubber circuit of a switching power supply, a TV horizontal resonance circuit, etc, and a capacitor using the dielectric ceramic composition.
2. Description of the Related Art
Like in other electronic devices, frequency is being rapidly increased and size rapidly decreased in a switching power supply or the like. Therefore, a capacitor used in a snubber circuit of the switching power supply is also strongly required to have a smaller size and higher frequency.
In the snubber circuit of the switching power supply, the waveform of a voltage loaded on the capacitor is a non-sinusoidal wave having a frequency of 50 to 150 kHz. It is known that such a non-sinusoidal wave having a constant frequency can be Fourier-expanded to a sinusoidal series containing a high-order frequency component. Namely, a non-sinusoidal wave (f) is represented by the following equation 1:
V(t)=V0+xcexa3{Vcnxc2x7cos(nxcfx89t)+Vsnxc2x7sin(nxcfx89t)}xe2x80x83xe2x80x83Equation 1 
wherein n represents a natural number of 1 or more, V0 represents a DC bias component, Vcn represents the amplitude of n-th cosine wave and Vsn represents the amplitude of n-th sinusoidal wave.
Actual Fourier expansion indicates that the non-sinusoidal wave has a waveform containing a high frequency of up to about 1 MHZ.
The apparent power (Pa) due to a voltage containing the harmonic wave is represented by the following equation 2:                     Pa        =                  ∑                      xe2x80x83                    ⁢                      {                                                            n                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                  Cn                                                                      1                    +                                                                  (                                                  tan                          ⁢                                                      xe2x80x83                                                    ⁢                          δ                                                )                                            2                                                                                  ·                                                                    Vc                    n                    2                                    +                                      Vc                    n                    2                                                  2                                                                        Equation        ⁢                  xe2x80x83                ⁢        2            
where tan xcex4 represents dielectric tangent.
FIG. 1 shows an equivalent circuit of a capacitor. In the equivalent circuit, self heating due to capacitor loss is substantially equal to the energy consumed by equivalent series resistance r, and is thus represented by the following equation 3:                     Q        ≈                                            tan              ⁢                              xe2x80x83                            ⁢              δ                                                      1                +                                                      (                                          tan                      ⁢                                              xe2x80x83                                            ⁢                      δ                                        )                                    2                                                              ·          Pa                                    Equation        ⁢                  xe2x80x83                ⁢        3            
Therefore, in order to decrease the self heating of the capacitor in the snubber circuit of the switching power supply or the like, on which the non-sinusoidal wave is loaded, the capacitor loss must be decreased in the frequency range of up to about 1 MHZ.
For the above-described reason, a circuit such as the snubber circuit of the switching circuit, or the like, to which a voltage containing a harmonic wave is applied, causes the problem of the self heating due to capacitor loss. Therefore, a capacitor using a dielectric material causing less loss is used.
For example, Japanese Examined Patent Publication No. 59-8923 discloses a low-loss dielectric material which contains Sr, Pb, Mg and Ti, and which has a dielectric constant of 1500 to 3000 at a measurement frequency of 1 kHz, and a dielectric tangent tan xcex4 of 0.0 to 0.3%.
Also Japanese Examined Patent Publication No. 8-15005 discloses a ceramic composition comprising a main component composed of Sr, Pb, Ca, Bi, Ti and Mn, and a glass component and Cu which are added to the main component.
Japanese Unexamined Patent Publication No. 2-133371 also discloses a ceramic composition comprising a main component composed of Sr, Pb, Ca, Bi, Ti and Mn, and a glass component and Cu which are added to the main component.
A capacitor comprising a conventional low-loss dielectric material has a problem in which use is limited by the self heating of the capacitor when a voltage containing a high harmonic wave is applied, and the problem of limiting miniaturization.
Namely, the dielectric ceramic composition disclosed in Japanese Examined Patent Publication No. 59-8923 has a dielectric tangent tan xcex4 at 1 MHZ of 5% or more, and the problem of limiting the use by the great self heating of the capacitor when a voltage containing a high harmonic wave is applied, as described above.
On the other hand, the dielectric ceramic composition disclosed in Japanese Examined Patent Publication No. 8-15005 has a dielectric constant of as low as about 500 to 1000 at a measurement frequency, and thus has the problem of limiting miniaturization of a capacitor.
In addition, inexpensive copper and nickel have recently been used as electrode materials for a ceramic capacitor instead of conventional versatile silver. Particularly, copper causes no electro-migration as a defect of a silver electrode, and thus exhibits high reliability and is relatively inexpensive. Particularly, in the method of forming a copper electrode by sputtering, a thin film electrode can be formed at low cost.
However, in forming the copper electrode by sputtering, the ceramic sintered body of a ceramic capacitor itself is exposed to an atmosphere under a high vacuum of about 10xe2x88x921 Pa during the formation of the electrode. However, the dielectric ceramic compositions disclosed in Japanese Examined Patent Publication No. 59-8923, Japanese Examined Patent Publication No. 8-15005, and Japanese Unexamined Patent Publication No. 2-133371 cause great self heating when used under a signal of a high voltage of several tens to several hundreds V, thereby causing the problem of a large loss.
Accordingly, it is an object of the present invention to solve the above problems of the conventional techniques and provide a dielectric ceramic composition which can be used in a high frequency region and a high voltage region with a low loss, which can suppress self heating to 20xc2x0 C. or less, and which can obtain a high dielectric constant, whereby permitting the advance in miniaturization of a capacitor and achieving desired characteristics even when an external electrode is formed by sputtering.
In order to achieve the object, a dielectric ceramic composition of the present invention comprises 100% by weight of a main component composed of about 35 to 55% by weight of SrTiO3, about 10 to 35% by weight of PbTiO3, about 5 to 12% by weight of CaTiO3, about 8 to 25% by weight of Bi2O3, and about 5 to 13% by weight of TiO2, and about 0.02 to 0.5% by weight of MnO, about 0.05 to 2% by weight of an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Dy, Er and Y, and about 0.02 to 0.8% by weight of CuO.
A capacitor of the present invention comprises a ceramic sintered body composed of the dielectric ceramic composition of the present invention, and a plurality of thin film electrodes formed on the outer surface of the ceramic sintered body and mainly composed of copper.
In a particular aspect of the present invention, a lead terminal in the capacitor is connected to each of the plurality of thin film electrodes.